This invention relates to controls for solid state electrical power conversion equipment, and particularly to a control system that, in combination with a solid state AC inverter, is capable of producing an ideal sinusoidal voltage waveform output regardless of load induced or switching harmonics.
Present day, solid state switching power inverters produce output voltage waveforms that are somewhat less than ideal sinusoidal in shape. This is in spite of the normally liberal use of harmonic filters at the inverter output and connection to balanced linear loads. The reason is the switching characteristic of the solid state power switches which are arranged in bridges. Depending on the number of poles and switching frequency, harmonic voltages are produced. These harmonic voltages are usually filtered out by L, C and R trap combinations which may be connected at the inverter output. Using this approach, the harmonic voltages can be reduced to a few percent of the fundamental. However, because of the harmonic voltages, the output voltage waveform will show steps or spikes, and will depart from the ideal sinusoidal shape.
If the power inverter has to supply non-linear loads, the waveform distortion becomes greater due to the feedback effect of many sharp swings in the load voltage and current. This distortion is very difficult to correct, using the established state of the art filter methods. Clearly, if a low distortion voltage waveform is specified under non-linear loads, a new approach to the filtering problem is needed. This new approach is provided by the control system invention described herein.
The invention is a control system for poly-phase power inverters that uses feedback sensed voltages and currents to control the on/off switching times of the power inverter switches in the bridges, creating a nearly ideal sinewave voltage output. The control system comprises line inductances and capacitors connected in a filter configuration in the inverter output power lines, voltage and current sense circuits, a load current compensation circuit, a reference sinusoidal voltage generator and an optimal controller.
The load current compensation circuit outputs signals that relate to the sensed load current and line inductance. The optimal controller utilizes signal inputs from the sense circuits, load current compensation circuit and reference sinusoidal voltage generator to produce switching signals to the power inverter driver signals that will result in an ideal sinusoidal voltage inverter output.
The optimal controller is digital, so that there is virtually no delay in implementing the feedback control, making it essentially instantaneous. The control system automatically corrects for any sensed line voltage distortions.
Accordingly, it is a principal object of this invention to provide a control system for poly-phase power inverters that will immediately correct for any line voltage distortions and produce an ideal sinusoidal voltage waveform at the power inverter output.
Another object is to provide a means of control that will provide a high recovery speed for output voltage transients.
Yet another object is to provide a control system that will eliminate DC components in the AC voltage output and decrease transmitted noise.
Further objects and advantages of the invention will become apparent from studying the following portion of the specification, the claims and the attached drawings.